Sputtering systems are widely used for depositing materials on workpieces, such as semiconductor wafers, display panels, mechanical parts, etc. Sputtering is sometimes referred to as physical vapor deposition, or PVD. In a sputtering operation, thin films comprising materials such as Al, Au, Cu, and Ta are deposited in a vacuum on the workpieces. It is common to deposit a stack of thin films of different materials, such as to form a tri-layer structure comprising Ti/Cu/Au.
Applicant had previously developed a PVD system having multiple wedge-shaped targets around a top wall of a circular vacuum chamber, where workpieces are mounted on a rotating circular pallet below the targets for being positioned under the different targets. The diameter of the pallet was approximately the same as the inner diameter of the vacuum chamber. The rotating pallet not only creates a more uniform deposition, but allows each workpiece to be positioned under targets of different materials for depositing a stack of different materials on the workpieces. This is described in Applicant's U.S. Pat. No. 7,682,495. However, with such a system, the center of the pallet could not be positioned under any of the targets. This was acceptable when the workpieces, such as silicon wafers, were mounted on the pallet away from the center so as to be fully under a target for sputtering. If the workpiece was a single large rectangular panel that overlaid the center of the rotating pallet, the center portion of the panel could not be positioned under a target by the rotating pallet, resulting in the non-coverage of the middle area by the targets.
Thus, what is needed is a technique for performing a PVD process on a large panel on a rotating pallet in a PVD chamber with multiple targets.
Further, for a large rectangular workpiece on the circular pallet, a portion of the targets must be directly above the corners of the rotating workpiece (where the corners extend near the outer perimeter of the pallet) for sputtering on the corner areas, but the sputtering material is wasted when the workpiece is not directly below the entire target, such as when a flat side of the rectangular workpiece (closer to the center of the pallet) is under only a portion of the target. Also, if multiple wafers are mounted on the pallet with spaces between them, sputtered material is wasted if it lands between the wafers.
Thus, what is also needed is a technique for performing a PVD process on a large panel (or other workpieces) on a rotating pallet in a PVD chamber where sputtering is only from portions of the targets that are directly above the workpiece.
The metal pallet is cooled using a liquid coolant flowing in the pallet. The pallet then cools flat wafers that are directly in contact with the pallet surface. However, for some uses, the workpiece is not in direct contact with the pallet surface and cannot be cooled by the pallet. Such is the case where the PVD system is used for depositing a metal layer over an array of IC packages for EMI shielding or for other applications where the workpiece is not in direct contact with the cooled pallet.
Thus, what is also needed is a technique for cooling workpieces supported by the pallet but are not in direct thermal contact with the pallet.
Other improvements are also described.